Many electrode assemblies of implantable medical devices, for the delivery of stimulation therapy and/or for diagnostic sensing, include fixation members configured to hold an electrode surface of the assembly in stable and intimate contact with tissue at an implant site. Those skilled in the art are familiar with electrode assemblies that include a helical structure; the helical structure typically includes a piercing distal tip to engage with tissue so that the helical structure may be screwed into the tissue and thereby fix an electrode surface of the assembly in intimate contact with the tissue. In some of these electrode assemblies the electrode surface is formed directly on the helical structure. Although such a helical structure can provide adequate fixation in many types of electrode assemblies, there is a need for new types of electrode assemblies and associated fixation members.